Being mean, I found the best way to do this was not to spend any money and do strengthening exercises every morning at home - currently doing a mixture of pilates and yoga, with crunchies (mmm!), press ups, squats (50lbs on a bar), curls and a few other bit'n'bobs . It does add getting on for 1/2 hour to my getting up routine, but no bad back for around 15 years...